Compression release-type engine braking or retarder systems are well-known in the art. Engine retarders temporarily convert an internal combustion engine of the compression ignition type into an air compressor in order to slow the engine. A compression release retarder decreases the kinetic energy of an engine by opposing the upward motion of the engine's pistons on the compression stroke. As a piston travels upward on its compression upstroke, the gases that are trapped in the cylinder are compressed. The compressed gases oppose the upward motion of the piston. When the piston nears the top of its stroke, an exhaust valve is opened to release the compressed gasses. After the pressure has been released from the cylinder, the piston cannot recapture the energy stored in the compressed gases on the subsequent expansion downstroke.
The braking system provides the operator with increased control over the vehicle. Properly designed and adjusted compression release-type engine retarders can generate retarding power equal in magnitude to a substantial portion of the power generated during positive power operations. Compression release-type retarders of this type supplement the braking capacity of the primary vehicle wheel braking system. Engine retarders may substantially extend the life of the primary wheel braking system of the vehicle.
The hydraulic valve control systems of compression release engine retarders have a number of components. A solenoid valve is typically provided to control the supply of engine oil to the hydraulic circuit of the compression release engine retarder. A master piston engages the hydraulic valve control system, typically at a rocker arm or cam. The master piston is linked to a slave piston through a hydraulic circuit. The slave piston is connected to an exhaust valve of the engine. When the compression release retarder is actuated, the rocker arm or cam lobe pushes against the master piston. The motion of the master piston is transferred to the slave piston through the hydraulic circuit, causing the slave piston to actuate and open the exhaust valve at a point near the end of the compression stroke.
Much of the potential energy created by compressing the gas in the cylinder is not recovered during the subsequent expansion or power stroke of the engine. Instead, it is dissipated through the exhaust and radiator systems of the engine. By dissipating the energy developed by compressing the cylinder charge, the compression release-type retarder slows the vehicle down.
The effectiveness of engine braking can be improved through the use of EGR. The exhaust gas may be recirculated into the cylinder at the time when the cylinder's piston is at or near Bottom Dead Center (BDC) at the beginning of the normal compression stroke. EGR allows a greater volume of air to be admitted to the cylinder. Consequently, the engine works harder compressing the denser air volume, and superior braking is achieved. EGR may also be employed during normal positive power operation. The benefits derived from positive power EGR are primarily reduced exhaust gas emissions.
Engine braking and EGR operations require that the cylinder exhaust valve be opened at times other than normal positive power openings. Engine braking requires the exhaust valves to be opened at or near Top Dead Center (TDC) at the completion of a cylinder's compression strike; EGR at or near Bottom Dead Center (BDC) at the beginning of the compression stroke. A typical engine's exhaust valve opening system holds the exhaust valve closed at these times.
An engine may include a retarding EGR event, a positive power EGR event, or both. These events maybe implemented as additional lobes on a cylinder's exhaust valve cam. A separate add-on system has been proposed to be mounted to the engine significantly increasing the externally installed dimensions. In addition, the exhaust valve cam may include a compression release brake lobe, as well as the main exhaust event lobe. Limited space on the exhaust valve cam may make it difficult to include lobes for retarding EGR and positive power EGR events, along with lobes for compression release braking and the main exhaust event. The valve lift profiles for the main exhaust event and the positive EGR event may overlap. Overlapping is undesirable because it may limit the engine's capability to achieve all of the desired events. As a result, there is a need to provide a valve actuation system which can accommodate multiple engine valve events.
Current valve actuation systems possess other deficiencies as well. In many internal combustion engines, the engine cylinder intake and exhaust valves are opened and closed by fixed profile cams, and more specifically, by one or more fixed lobes which are an integral part of each of the cams. The use of fixed profile cams makes it difficult to make the necessary adjustments to the timing and/or amount of engine valve lift for various engine operating conditions, such as different engine speeds.
One method of adjusting valve timing and lift has been to incorporate a "lost motion" device in the linkage between the valve and the cam. Lost motion is the term applied to a class of technical solutions for modifying the valve motion proscribed by a cam profile with a variable length mechanical, hydraulic, or other linkage means. A cam lobe provides the maximum (longest dwell and greatest lift) motion needed over a full range of engine operating conditions. In a lost motion system, a variable length system is included in the valve train linkage to subtract or lose part or all of the motion imparted by the cam to the valve.
This variable length system may, when expanded fully, transmit all of the cam motion to the valve, and when contracted fully, transmit none or a minimum amount of the cam motion to the valve. An example of such a system and method is provided in Hu, U.S. Pat. Nos. 5,537,976 and 5,680,841, which are assigned to the same assignee as the present application, and which are incorporated herein by reference.
In the lost motion system of U.S. Pat. No. 5,680,841 (the "'841 patent"), an engine cam shaft actuates a master piston which displaces fluid from its hydraulic chamber into a hydraulic chamber of a slave piston. The slave piston in turn acts on the engine valve to open it. The lost motion system may be a solenoid valve and a check valve in communication with the hydraulic circuit including the chambers of the master and slave pistons. The solenoid valve may be maintained in a closed position in order to retain hydraulic fluid in the circuit. As long as the solenoid valve remains closed, the slave piston and the engine valve respond directly to the motion of the master piston, which in turn displaces hydraulic fluid in direct response to the motion of a cam. When the solenoid is opened temporarily, the circuit may partially drain, and part or all of the hydraulic pressure generated by the master piston may be absorbed by the circuit rather than be applied to displace the slave piston.
Prior lost motion systems have typically not utilized high speed mechanisms capable of rapidly varying the length of the lost motion system. These lost motion systems have not been capable of assuming more than one length during a single cam lobe motion, or even during one cycle of the engine. The use of a high speed mechanism to vary the length of the lost motion system allows for more precise control over valve actuation, and accordingly optimal valve actuation may be attained for a wide range of engine operating conditions.
As discussed above, engine efficiency and performance may be maximized through the use of variably timed positive and negative power EGR events. Similarly, braking performance may be enhanced through two-cycle braking. A lost motion system may be used to implement these operations. In a lost motion system, working fluid is drained and added at precise times to the hydraulic link between the master piston and slave piston. The engine valve profile may be modified by modifying the motion of the master piston, which follows a cam, prior to its transfer to the slave piston. In this way, variable timing is achieved. Variable timed positive and negative power EGR, as well as two-cycle braking, may be difficult to achieve on an exhaust valve cam already crowded with a main exhaust event lobe and a compression release brake event lobe because of inadequate base circle "residence time." Residence time refers to the amount of time at which the cam presents a zero lift profile to the cam follower or master piston. This time is generally proportional to the amount of space on the cam not taken up by different lobes.
An example of a lost motion system and method used to obtain retarding and exhaust gas recirculation is provided by the Gobert, U.S. Pat. No. 5,146,890 (Sept. 15, 1992) for a Method And A Device For Engine Braking A Four Stroke Internal Combustion Engine, assigned to AB Volvo, and incorporated herein by reference. Gobert discloses a method of conducting exhaust gas recirculation by placing the cylinder in communication with the exhaust system during the first part of the compression stroke and optionally also during the latter part of the inlet stroke. Gobert uses a lost motion system to enable and disable retarding and exhaust gas recirculation, but such system is not variable within an engine cycle.
The '841 patent discloses an internal combustion engine with valves that are opened by cams cooperating with hydraulic circuits that are partly controlled by electrically operated hydraulic fluid valves. The system of the '841 patent is limited by inadequate residence time since its system includes the use of a single cam for controlling all openings of the engine valve regardless of the engine's mode of operation.
As a result of the shortcomings of existing engine valve actuation systems. There is a need for a system which may accommodate all valve events necessary for efficient engine operation, including EGR, compression release braking and positive power operations.